This invention relates to power supply circuits, and more particularly to a power supply circuit which reserves a power supply for a memory element in an electronic tuning type receiver.
Recently, a variety of receivers in electronic tuning systems have been proposed due to remarkably rapid progress in electronics. In the electronic tuning system, a receiving frequency is controlled by voltage-controlling a variable capacitance diode (varicap) provided in a tuning section. There have been proposed a variety of systems for controlling the tuning voltage. A typical one of the systems is a digital system in which, for purposes of stability, a digital value is employed for indication and the digital value is subjected to digital-to-analog conversion to be emplyed as a tuning control voltage. In an electronic tuning type receiver thus organized, a preset station selecting circuit is provided in which digital values manually set up, are stored in a read-write memory separately according to bands and channels, and the values thus stored are read out for station selection. Generally, a memory element is of the volatile type or non-volatile type.
In the above-described electronic tuning type receiver with a suitable volatile type memory element, it is necessary to provide a power supply for the memory element for allowing the latter to maintain its memory contents at all times even when the power switch is turned off. This requirement can be readily met if a commercial power supply is utilized for the receiver; however, it is difficult to satisfy the requirement in the case where the receiver is used as a receiver on a vehicle as in the case of a car stereophonic receiver. In other words, if the engine switch is turned off, all of the power supplies are turned off. Therefore, the memory element cannot maintain its memory operation and, accordingly, the preset station selection cannot be conducted. Furthermore, when the engine of an automobile is started, the starter consumes a great amount of electric power, as a result of which the supply voltage is decreased and, therefore, it is not possible for the memory element to carry out its memory operation.